Aprotic heterocyclic anion triazolide ionic liquids - A new class of ionic liquid anion accessed by the Huisgen cycloaddition reaction

Article abstract of DOI:10.1055/s-0033-1338435

The triazole core is a highly versatile heterocyclic ring which can be accessed easily with the Cu(I)-catalyzed Huisgen cycloaddition reaction. Herein we present the preparation of ionic liquids that incorporate a 1,2,3-triazolide anion. These ionic liqui

Full text of DOI:10.1055/s-0033-1338435

LETTER
▌1093
lAettper rotic Heterocyclic Anion Triazolide Ionic Liquids – A New Class of Ionic
Liquid Anion Accessed by the Huisgen Cycloaddition Reaction
Aprotic Heterocyclic Anion Triazolide Ionic Liquids
Robert L. Thompson,*a,b Krishnan Damodaran,c David Luebke,a Hunaid Nulwala*a,d
a
National Energy Technology Laboratory, 626 Cochrans Mill Rd., Pittsburgh, PA 15236, USA
Fax +1(412)3864542; E-mail: Robert.thompson@contr.netl.doe.gov
b
URS Corp., P. O. Box 618, South Park, PA 15129, USA
c
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
d
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
E-mail: Hnulwala@andrew.cmu.edu
Received: 27.02.2013; Accepted after revision: 28.03.2013
the ability to tune the electronics of the anion via ring sub-
Abstract: The triazole core is a highly versatile heterocyclic ring
stitution.
which can be accessed easily with the Cu(I)-catalyzed Huisgen cy-
cloaddition reaction. Herein we present the preparation of ionic liq-
uids that incorporate a 1,2,3-triazolide anion. These ionic liquids
were prepared by a facile procedure utilizing a base-labile pivaloyl-
methyl group at the 1-position, which can act as precursors to 1H-
4-substituted 1,2,3-triazole. These triazoles were then subsequently
converted into ionic liquids after deprotonation using an appropriate
ionic liquid cation hydroxide. The densities and thermal decompo-
sitions of these ionic liquids were measured. These novel ionic liq-
uids have potential applications in gas separations and in metal-free
catalysis.
Substituted 1,2,3-triazoles can serve as AHA IL precur-
sors and fortuitously possess a pKa amenable to the forma-
tion of an anion which could react reversibly with CO2.
Azoles which are too basic tend to interact too strongly
with H2O and bind CO2 irreversibly; those which are not
basic enough do not interact with CO2 strongly enough to
bind it. The pKa of 1,2,3-triazole falls in an intermediate
range, where these two trends may tend to balance each
other out.
Key words: ionic liquids, triazoles, triazolides, aprotic heterocyclic
anions, CO2 capture, click chemistry
We have prepared triazolide-based ILs utilizing a simple
Cu(I)-catalyzed cycloaddition reaction,12 which is capable
of generating a variety of ILs with multiple functional
groups located on the 1,2,3-triazole core. The versatility
and ease of access of triazole ring has found its use as an-
tifungal agents in medicinal chemistry,13 in polymer
chemistry,14 and in ILs.15 The general reaction scheme is
shown in Scheme 1. The Huisgen cycloaddition reaction
between azide and alkyne is highly tolerant of varying the
functional group and requires mild reaction conditions,
yielding essentially pure 1,4-difunctional regioiso-
mers.15a,16 When the azide used possesses a labile group
such as pivaloylmethyl (POM), the resulting triazole can
easily be transformed into the protonated 4-substituted tri-
azole, which in turn is a facile precursor for a large family
of ILs.
Ionic liquids (ILs) are organic salts which are liquids at
room temperature.1 ILs possess attractive properties in-
cluding unique solubilities,2 wide electrochemical win-
dows, negligible vapor pressures, and good thermal
stabilities3 and has resulted in its use in various applica-
tions including use as solvents,4 as catalysts,5 and in gas
separations.6
The use of aprotic heterocyclic anions (AHA) in ILs, orig-
inally reported by Ogihara,7 has recently been reported by
Wang to efficiently and reversibly capture CO2 in equi-
molar stoichiometry.8 They report that IL prepared from
some simple azoles can be tuned for stability, CO2 absorp-
tion enthalpy, and CO2 capacity based on the pKa of the
azole. AHA-type ILs have also been shown by Gurkan9 to
be a successful general approach to selectively and revers-
ibly react with CO2 without suffering detrimental increas-
es in viscosity.10 Inspection of the relative pKa of
tetrazole, 1,2,3-triazole, and 1,2,4-triazole (8.2, 13.9, and
14.8, respectively, in DMSO),11 shows that 1,2,3-triazole
possesses an intermediate pKa; Wang reported that tetra-
zole did not successfully capture CO2, whereas 1,2,4-tri-
azole did. This suggests that 1,2,3-triazoles would be
viable candidates for CO2 capture, particularly in light of
.
R2
R2 N3
OH–
H+
Cu(I)
N
+
N
R1
N
R1
[P4444]OH
H
N–
N
N
+
[P4444
]
N
N
– H2O
R1
N
R1
R1 = Ph, C6H13, t-Bu, Me3SiCH2,
F3C(CF2)5(CH2)2OCH2; R2 = POM
Scheme 1 Synthetic approach used in this work
SYNLETT 2013, 24, 1093–1096
Advanced online publication: 18.04.2013
DOI: 10.1055/s-0033-1338435; Art ID: ST-2013-S0183-L
© Georg Thieme Verlag Stuttgart · New York
In this work, we present the synthesis of novel triazolide-
based AHA ILs. The 1,4-disubstituted precursor struc-
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

1094
R. L. Thompson et al.
LETTER
N–
N–
tures prepared are shown in Figure 1. These molecules
were converted into 1H-4-substituted-1,2,3-triazoles, as
shown in Figure 2, after alkaline hydrolysis in a metha-
nol–water mixture and were converted into ILs by treat-
ment with tetrabutyl phosphonium hydroxide. The 1H-4-
substituted 1,2,3-triazole and hydroxide undergo an acid–
base reaction to form the IL product. The AHA ILs pre-
pared in this work are shown in Figure 3.
N
N
1c
N
2c
N
P+
P+
N–
N
4c
Si
N
P+
N–
O
N
N
O
N
3c
P+
N
N
O
2a
O
O
1a
N
N
N
N
N
O
F
F
F
N
F
F
F
3a
F
P+
N–
F
F
F
F
F
O
F
N
O
5c
N
N
O
N
4a
N
F
Si
Figure 3 The tetrabutyl phosphonium 4-substituted-1,2,3-triazolide
ILs 1c–5c synthesized
F
F
F
F
F
F
F
F
F
3b). A silyl-substituted and fluorinated ether-substituted
triazole (4b, 5b) were also included to observe the effect
of hydrophobic substituents on the AHA IL.
F
O
F
O
O
5a
F
N
N
N
The physical properties of the resulting AHA ILs were
measured, the results are listed in Table 1. The densities
(ρ) of all of the ILs, except for the fluorinated 5c, were ap-
proximately 1 g/mL; the molar volumes (Vm) listed were
calculated from these density values. Karl Fisher (KF) ti-
tration of the ILs revealed that after vacuum drying to con-
stant weight at 50 °C, 0.9–2.4 wt% water still remained in
the ILs. In addition, preliminary experiments measuring
the viscosities and CO2 absorption abilities were con-
ducted (see Supporting Information).
Figure 1 The 1-POM-4-substituted-1,2,3-triazoles 1a–5a synthe-
sized
N
H
N
N
N
N
2b
N
F
H
1b
F
F
H
F
N
F
N
N
N
F
F
3b
F
F
N
N
Tonset: 3c > 2c > 4c > 1c > 5c
F
F
N
Si
H
H
F
electron-donating ability: 3c ~ 2c > 4c > 1c > 5c
F
O
4b
5b
N
Scheme 2 Comparison of Tonset and electron-donating ability for ILs
1c–5c
N
Figure 2 The 1H-4-substituted-1,2,3-triazoles 1b–5b synthesized
Thermal analysis of the decomposition of the ILs 1c–5c
shows that electron-donating groups (1c, 5c) on the tri-
azole ring begin to decompose (Tonset) at lower tempera-
tures than those with electron-withdrawing groups (2c–
4c). Alkyl groups on 2c and 3c cause the ILs to begin ther-
mal decomposition about 75 °C cooler than for 1c and 5c.
A comparison of Tonset and substituent electron-donating
ability for ILs 1c–5c is shown in Scheme 2. The onset of
thermal decomposition for ILs 1c–5c were found to be
comparable with results reported by Wang, et al. for
The IL formation can be extended to any other IL cation
by using the appropriate hydroxide. By varying the elec-
tronic nature of the substituent at the 4-position on the tri-
azole ring, stabilization of the negative charge on the
resulting anion can be affected and increased molar
volume can be achieved, which is important for solvent
applications. Access to these new triazole precursors are
granted by the exceptionally versatile cycloaddition
reaction.
[P66614][imidazolide] and [P66614][pyrazolide] (Tdec
=
A set of five AHA ILs based on 4-substituted-1,2,3-tri-
azole was synthesized and characterized. The triazoles
prepared were chosen to include a variety of substituents,
with both electron-withdrawing (Ph: 1b) and electron-
donating groups at the 4-position (n-Hex and t-Bu: 2b and
252 °C and 182 °C, respectively).8
The relative effect of electron-donating and withdrawing-
1
substitution at the 4-position is also observed in the H
NMR chemical shift of the triazole ring proton at the 5-po-
Synlett 2013, 24, 1093–1096
© Georg Thieme Verlag Stuttgart · New York

LETTER
Aprotic Heterocyclic Anion Triazolide Ionic Liquids
1095
Table 1 Physical Properties of Triazolide ILs 1c–5c
IL
FW
(g/mol)
ρa
(g/mL)
Vm
(mL/mol)
Tonset
(°C)
H2O
(wt%, by KF)
H2O
(mol%, by KF) (CO2/mol IL) (cP)
Mol absorbed ηa
1c
2c
3c
4c
5c
403.58
411.65
383.59
413.70
704.61
0.9836
0.9324
0.9290
0.9400
1.2141
402.7
431.2
401.2
416.6
578.6
238
185
151
227
263
1.06
23.7
53.7
19.5
23.9
38.0
0.10
0.40
0.07
0.30
0.23
2255
1357
2414
1659
8275
2.35
0.914
1.04
0.973
a Densities and viscosities were measured at ambient temperature (19–24 °C) and were not controlled.
sition. The shifts in 1,2,3-triazolides with electron-donat- the physical properties and reactivity of the resulting
ing groups in 2c, 3c, and 4c (δ = 7.12–7.23 ppm in CDCl3) 1,2,3-triazolide anions is realized, depending upon the
are observed upfield from those of the electron-withdraw- electronic nature and steric bulk of the substituent. The
ing groups in 1c and 5c (δ = 7.36–7.59 ppm), reflecting electronic changes in the azole rings are reflected in the
1
the less shielded environment for the protons in the latter chemical-shift changes observed in the H NMR spectra
cases. Related effects are also reflected in the relative of the azole ring protons. Characterization of these new
chemical shifts for the H5 proton in the 1H-1,2,3-triazoles ILs by mass spectrometry confirms their compositions.
1b–5b and in the 1,2,3-triazolide ILs 1c–5c, depicted in Current work is under way to study the viscosity of these
Figure 4. The H5 signal shifted upfield 0.1–0.4 ppm for all ILs and to study their reactivity with H2O and CO2 using
five triazoles after conversion into ILs, indicating that the both laboratory experiments and computational chemis-
electronic environment of the azole ring was more elec- try.
tron rich after being converted into anionic form.
General Method for the Synthesis of 1-POM-4-Phenyl-1,2,3-tri-
azole (1a)
Azidomethyl pivalate was synthesized from NaN3 and chlorometh-
yl pivalate in H2O according to literature procedures.17 Azidometh-
yl pivalate (5.00 g, 31.8 mmol), phenyl acetylene (4.12 g, 40.4
mmol), Et3N (3.63 g, 35.9 mmol), and 3% Cu/charcoal (3.05 g, 1.44
mmol) were placed in a Schlenk tube, dissolved in dioxane (15 mL),
and heated overnight at 80 °C. This mixture was filtered to remove
the catalyst, and then evaporated and vacuum dried to give a brown
solid. The residue was dissolved in minimal Et2O, filtered through
a 0.2 μm syringe filter, evaporated, and vacuum dried to give 1a as
a white solid (7.25 g, 28.0 mmol, 88% yield). 1H NMR (300 MHz,
CDCl3): δ = 8.04 (s, 1 H, triazole CH), 7.87 (m, 2 H, 2-Ph), 7.45 (m,
2 H, 3-Ph), 7.42 (m, 1 H, 4-Ph), 6.29 (s, 2 H, piv-CH2), 1.21 (s, 9 H,
piv-CH3). 13C NMR (75.5 MHz, CDCl3): δ = 178.0 (C=O), 148.3
(triazole), 130.0 (triazole), 128.9, 128.5, 125.8, 120.9, 69.7, 38.8,
26.8. FTIR (ATR film): 1737 (C=O) cm–1. MS: m/z calcd for
C14H18N3O2+ [M + H]+: 260; found: 260.
Figure 4 The 1H NMR chemical shifts (in CDCl3) for triazoles 1b–
5b and triazolide ILs 1c–5c
General Method for the Synthesis of 1H-4-Phenyl-1,2,3-triazole
(1b)
The general procedure for removal of the methyl pivalate leaving
group was based on that described by Loren et al.17 The structures
for all molecules synthesized by this method are shown in Figure 3.
KOH (7.46 g, 133 mmol) and 1a (7.25 g, 28.0 mmol) were stirred
in MeOH–H2O (50 mL, 1:1) in air for 2 h at r.t., then was neutral-
ized with 1 M HCl (100 mL, 100 mmol) to form a cloudy white so-
lution. The mixture was filtered and the solids were rinsed with H2O
(300 mL) and vacuum dried to give off-white solids (3.48 g, 24.1
mmol, 86% yield). 1H NMR (400 MHz, CDCl3): δ = 8.03 (s, 1 H,
triazole CH), 7.83 (m, 2 H, 2-Ph), 7.47 (m, 2 H, 3-Ph), 7.40 (m, 1 H,
4-Ph). 13C NMR (101 MHz, CDCl3): δ = 148.9 (triazole), 132.2 (tri-
azole), 129.0, 128.9, 126.2, 126.1. FTIR (ATR film): 3159, 3114,
2845, 1466, 1453, 1131, 1082, 1003, 972, 915, 873, 764, 693, 517
The mass spectra of 1,2,3-triazolide IL solutions in meth-
anol were all obtained, and the results are listed in the
Supporting Information (SI Table 1). In all five cases, the
molecular ion for the tetrabutyl phosphonium cation was
observed in the positive mode, and the molecular ion for
the triazolide anion was observed in the negative mode,
confirming the identities of all products.
In conclusion, we have synthesized a series of five novel,
tunable ILs based on substituted-1,2,3-triazoles, prepared
via Huisgen cycloaddition chemistry. These AHA ILs
possess densities and thermal stabilities which are compa-
rable to other AHA ILs cited in the literature.7–9 By careful
selection of the substituent at the 4-position, control over
+
cm–1. ESI-MS (+/–, MeOH): m/z calcd for C8H8N3 [M + H]+:
146.17; found: 145.95.
General Method for the Synthesis of Tetrabutyl Phosphonium
4-Phenyl-1,2,3-triazolide ([P4444][4-Ph-1,2,3-TZ], 1c)
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1093–1096

1096
R. L. Thompson et al.
LETTER
The general procedure for the synthesis of 1,2,3-triazolide IL was
derived from that described by Fukumoto.18 The structures for all IL
synthesized by this method are shown in Figure 4. A solution of 1b
(1.63 g, 11.2 mmol) in EtOH (25 mL) was treated with 40% tetra-
butyl phosphonium hydroxide ([P4444]OH) in H2O (7.84 g, 11.3
mmol) and was stirred at 50 °C for 6 h. The solvent was then evap-
orated, and the residue was vacuum dried at 50 °C until constant
weight. The product was taken up in EtOAc (10 mL), filtered
through a 0.2 μm syringe filter, evaporated, and vacuum dried to
give a brown liquid (4.81 g, 106% yield). This liquid was then pu-
rified as described by Burrell,19 by refluxing over activated charcoal
in MeOH at 65 °C for 24 h. Evaporation and vacuum drying at
50 °C gave a pale red-brown liquid. 1H NMR (700 MHz, DMSO):
δ = 7.69 (m, 2 H, 2-Ph), 7.60 (s, 1 H, triazole CH), 7.26 (m, 2 H, 2-
Ph), 7.04 (m, 1 H, 4-Ph), 2.16 (m, 8 H, PCH2), 1.41 (m, 16 H,
PCH2), 0.90 (t, 12 H, PCH3). 13C NMR (101 MHz, DMSO): δ =
142.8 (triazole), 135.7 (triazole), 128.1, 126.9, 124.4, 124.3, 23.2
(dd, PCH2), 17.3 (d, 48.5 Hz, PCH2), 13.2 (PCH3). 31P NMR (162
MHz, DMSO): δ = 33.6. FTIR (ATR film): 2958, 2928, 2871, 1603,
1465, 1379, 1096, 1047, 965, 906, 763, 718, 696, 682, 605, 512 cm–1.
ESI-MS (+/–, MeOH): m/z calcd for C22H43N3P: 403.58; m/z calcd
for cation C16H36P+ [M+]: 259.43; found: 259.27; m/z calcd for an-
ion C8H6N3– [M–]: 144.15; found: 144.07.
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Disclaimer
This project was funded by the Department of Energy, National En-
ergy Technology Laboratory, an agency of the United States Gov-
ernment, through
a support contract with URS Energy
&Construction, Inc. Neither the United States Government nor any
agency thereof, nor any of their employees, nor URS Energy &
Construction, Inc., nor any of their employees, makes any warranty,
expressed or implied, or assumes any legal liability or responsibility
for the accuracy, completeness, or usefulness of any information,
apparatus, product, or process disclosed, or represents that its use
would not infringe privately owned rights. Reference herein to any
specific commercial product, process, or service by trade name,
trademark, manufacturer, or otherwise, does not necessarily consti-
tute or imply its endorsement, recommendation, or favoring by the
United States Government or any agency thereof. The views and
opinions of authors expressed herein do not necessarily state or re-
flect those of the United States Government or any agency thereof.
Acknowledgment
This technical effort was performed in support of the National En-
ergy Technology Laboratory’s ongoing research under the RES
contract DE-FE0004000. The authors would like to acknowledge
Dr. Erik Albenze for assistance in performing CO2 absorption expe-
riments and Dr. Brian Kail and Kimberly Carter for their support in
providing TG and MS analyses.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
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Synlett 2013, 24, 1093–1096
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